The present invention relates to epoxy functional polyester resins, having an increased molecular weight, to a process for their preparation and to outdoor durable powder coating compositions comprising them.
Triglycidylesters which can be used in good quality outdoor durable coatings and in moulding compositions are disclosed in European Patent Application No. 447360A (EP-A-447,360). Due to the anhydride half ester nature of the terminal carboxyl functions present in the tricarboxylic acid adduct precursors, strong alkaline conditions should be avoided during glycidation of these tricarboxylic acid adducts to avoid hydrolysis of the glycidylester formed and/or hydrolysis of one or more ester groups in the resin backbone. As a result thereof the triglycidylester produced will contain a relatively high level of hydrolyzable chlorine and/or will contain low molecular weight hydrolysis products which might cause toxicity problems, as may be derived from "Water based coatings with excellent saponification stability," XIIIth Int. Conf. 1987, Athens, Greece, p. 175.
The high level of hydrolyzable chlorine is reflected in Example 2 of EP-A-447,360 which relates to the glycidation of the 2:1 adduct of hexahydrophthalic anhydride and dimethylolpropionic acid. The product obtained has a chlorine content of 1.5%. Such a high level of residual chlorine is generally undesirable in coating compositions. In addition, due to the fact that the triglycidylesters reported in EP-A-447,360 are liquid, they can not be applied in powder coating compositions.
In International Application WO 96/11238, it was taught to a person skilled in the art of this specific area of curable coatings, that epoxy resins containing cycloaliphatic nuclei had the disadvantage that they could only provide brittle coating films when cured. This brittleness made them unsuitable for coating applications, as brittleness often led to poor adhesion.
European patent application No. 634434A2 discloses a process for the preparation of linear tertiary aliphatic carboxyl functional polyester resins, by reacting:
(a) at least one compound A' comprising one mono-functional primary- or secondary hydroxyl group and/or at least one compound A" comprising one primary- or secondary hydroxyl group and one tertiary aliphatic carboxyl group; PA1 (b) at least one aromatic or cycloaliphatic dicarboxylic acid compound B comprising two aromatic- or secondary aliphatic carboxyl groups or the anhydride thereof; PA1 (c) at least one diol compound C comprising two aliphatic hydroxyl groups, which may independently be a primary or a secondary hydroxyl group; and PA1 (d) at least one dihydroxymonocarboxylic acid compound D comprising a tertiary aliphatic carboxyl group and two aliphatic hydroxyl groups, which may each independently be primary or secondary hydroxyl, PA1 a) at least one aromatic and/or cycloaliphatic carboxylic acid compound A comprising two aromatic- and/or secondary aliphatic carboxyl groups or the anhydride thereof, PA1 b) at least one hydroxyl compound B comprising two aliphatic hydroxyl groups, which groups each independently may be primary or secondary hydroxyl groups, PA1 c) at least one hydroxyl substituted carboxylic acid compound C comprising at least one tertiary aliphatic carboxyl group and two aliphatic hydroxyl groups, which groups each independently may be primary or secondary hydroxyl groups, and PA1 d) optionally one carboxylic acid compound D comprising one carboxyl group, PA1 (a) at least one aromatic or cycloaliphatic dicarboxylic acid compound A comprising two aromatic- or secondary aliphatic carboxyl groups or the anhydride thereof; PA1 (b) at least one diol compound B comprising two aliphatic hydroxyl groups, which may independently be a primary or a secondary hydroxyl group; and optionally PA1 (c) compound C1 comprising one monofunctional primary- or secondary hydroxyl group and/or at least one compound C2 comprising one primary- or secondary hydroxyl group and one tertiary aliphatic carboxyl group; and optionally PA1 (d) a dihydroxymonocarboxylic acid compound D comprising PA1 (e) a trihydroxyalkane (E1) or tetrahydroxyalkane (E2), PA1 a) at least one compound of the formula ##STR1## wherein a.gtoreq.1 wherein R.sub.1 and R.sub.2 each may represent an alkyl group having from 1 to 4 carbon atoms, or wherein R.sub.1 and R.sub.2 may form together with the group ##STR2## a cycloalkyl group, which preferably represents 1,4-cyclohexane dicarboxylic acid (A1), optionally mixed with minor amounts of a corresponding compound of formula V, wherein a=0 or anhydride thereof (A2), PA1 b) at least one diol compound B comprising two aliphatic hydroxyl groups which may each independently be a primary or a secondary hydroxyl group; PA1 c) optionally a dihydroxymonocarboxylic acid compound C, comprising a tertiary aliphatic carboxyl group and two aliphatic hydroxyl groups, which may each independently be primary or secondary hydroxyl; and PA1 (d) optionally a trihydroxyalkane (E1) or tetrahydroxyalkane (E2),
the molar ratio of compounds A':A":B:C:D being EQU M:N:X+Y+1:X:Y PA2 wherein M+N=2, X ranges from 2 to 8 and Y ranges from 2-N to 8, at a temperature of from 100 to 240.degree. C., until essentially all the non-tertiary carboxyl groups as initially present in the reaction mixture have been reacted. PA2 the molar ratio of compounds A:B:C:D being EQU (X+Y-1):X:Y:Z, PA2 wherein X ranges from 2 to 8, Y ranges from 2 to 8, and Z ranges from 0 to 2. PA2 a tertiary aliphatic carboxyl group and two aliphatic hydroxyl groups, which may each independently be primary or secondary hydroxyl; and optionally PA2 the molar ratio of compounds A:B:C1:C2:D:E1:E2 being EQU X+Y+1:X:M:N:Y:Z:Q PA2 wherein M+N is in the range of from 0 to 2, X ranges from 2 to 8 and Y ranges from 0 to 8, Z ranges from 0 to 2 and Q ranges from 0 to 2 at a temperature of from 100 to 220.degree. C., until essentially all the non-tertiary carboxyl groups as initially present in the reaction mixture have been reacted, having a WPE in the range of from 250 to 800, with (IV) a carboxy functional polyester resin derived from the herein before mentioned component (a) and (b), or with a component (a) alone, in the presence of a catalyst. PA2 the molar ratio of compounds (A.sub.1 +A.sub.2):B:C:E1:E2 being X+Y+2Z+3Q+P:X:Y:Z:Q, wherein X ranges from 1 to 8, Y ranges from 0 to 8, Z ranges from 0 to 1 and Q ranges from 0 to 1 and wherein P ranges from 1 to 5, and preferably 1-3 and is most preferably equal to 1, at a temperature of from 100 to 220.degree. C., and preferably from 180 to 210.degree. C. if any compound C is present, until essentially all the hydroxyl groups as initially present in the reaction mixture have been reacted.
Moreover in this application were disclosed polyglycidylester resins obtainable by reacting said linear tertiary aliphatic carboxyl functional polyesters with an excess epihalohydrin in the presence of a suitable base and optional catalyst. Preferably, the polyesters were reacted with epichlorohydrin. Both the specified linear polyesters and the corresponding polyglycidylesters derived therefrom were used with a cross-linking agent for powder coating compositions.
In European patent application No. 720997A2, linear tertiary carboxyl functional polyesters and epoxy functional polyester resins are disclosed where these polyester resins were produced by reacting:
These polyester resins could be used together with a suitable curing agent for the production of powder coatings, or could be converted into the corresponding glycidylesters, which in combination with a suitable curing agent could be used for the production of powder coatings.
Although the linear tertiary aliphatic carboxyl functional polyester resins and the polyglycidylesters thereof enabled a certain progress as to the requirements of excellent outdoor durability (UV stability) and resistance against hydrolysis in the cured state with reference to their use in modern economically applied powder coatings, there is still a need for further improvement of this combination of properties.
On the other hand, novel powder coating binders for the exterior durable powder coating market derived from carboxylated polyester resins, cured with epoxy functional acrylate polymers, have been proposed during the Waterborne, Higher Solids and Powder Coatings Symposium, Feb. 5-7, 1997, New Orleans La., USA, T Agawa and E D Dumain, p. 342-353, "New Two-component Powder Coating Binders: Polyester acrylate hybrid as TGIC Cure Alternative.
However, as indicated on page 353, further improvements have to be made to provide smoother films, lower cure temperatures and UV durability to rival that of automotive topcoating or outdoor building panel topcoating.
The epoxy functional polyester resins obtainable according to the hereinbefore discussed documents, although showing attractive combinations coating properties, such as outdoor durability, flexibility, hardness, chemical resistance, could not meet the present storage stability requirement from the coating industry to powder coating compositions, comprising said epoxy functional polyester resins, to enable a conveniently handling, i.e. non-blocking or caking of the powder coatings when stored at temperatures up to 40.degree. C.
In connection with said storage stability, the epoxy functional polyester resins aimed at have to show an increased Tg (i.e. &gt;40.degree. C.). One of the possibilities of increasing the Tg of epoxy functional polyester resins is to increase the molecular weight (MW) or the epoxy equivalent weight (WPE).
However, the intermediate carboxyl functional polyesters to be initially prepared as starting materials for the direct glycidation process for the preparation of the corresponding epoxy functional polyester resins having the increased MW will show a viscosity under the preparation process conditions which is unacceptable high (e.g. &gt;80 poise at 200.degree. C.).
Moreover, epoxy functional polyester resins which might have been obtained by a direct glycidation of carboxyfunctional polyester resins, which might show an even slightly increased MW, should inevitably have such a high viscosity that the further finishing, i.e. removal of used solvents and low molecular weight by-products (devolatilization), will be impossible.
Therefore it is an object of the present invention to provide a process for the preparation of linear, epoxy functional polyesters, which show an acceptable storage stability at temperatures up to 40.degree. C. and which therefore show an increased Tg (over 40.degree. C.).
It is another object of the present invention to provide powder coating compositions, which comprise said epoxy functional polyesters aimed at, and which combine the attractive coating properties with an acceptable storage stability.